L2, the minor capsid protein of papillomavirus

Abstract

The capsid protein L2 plays major roles in both papillomavirus assembly and the infectious process. While L1 forms the majority of the capsid and can self-assemble into empty virus-like particles (VLPs), L2 is a minor capsid component and lacks the capacity to form VLPs. However, L2 co-assembles with L1 into VLPs, enhancing their assembly. L2 also facilitates encapsidation of the ∼8 kbp circular and nucleosome-bound viral genome during assembly of the non-enveloped T=7d virions in the nucleus of terminally differentiated epithelial cells, although, like L1, L2 is not detectably expressed in infected basal cells. With respect to infection, L2 is not required for particles to bind to and enter cells. However L2 must be cleaved by furin for endosome escape. L2 then travels with the viral genome to the nucleus, wherein it accumulates at ND-10 domains. Here, we provide an overview of the biology of L2.

Keywords: Daxx; E2; Encapsidation; Furin; HPV; Infection; L1; L2; Minor capsid protein; ND-10; Papillomavirus; SP100.

Figure 1. Diagram of known neutralizing epitope regions and protein interaction domains of HPV16 L2. Regions that were discovered using other PV L2 types are indicated

Diagram was adapted and updated from (Karanam et al., 2009)). See Table 1 for a full list of neutralizing antibodies.

Figure 2. Arrangement of L1 and L2 in HPV16 capsids

3D reconstructions derived by comparing cryo-electron micrograph images of HPV16 capsids made of L1 + L2 versus L1 only (Buck et al., 2008; Buck and Trus, 2012) (A). Interior of L1+L2 capsid without DNA and histones (B). Arrangement of L2 density (red) areas superimposed on the interior view of L1 only capsid (in blue) (C). Clustal analysis of L1 binding domain of L2 showing several conserved proline (PxxP) motifs (D). Clustal Analysis was performed with the UCSF Chimera package. Images were provided by the courtesy of both Christopher Buck and Benes Trus, NCI.

Figure 3. A model depiction of the early events of PV infection in the cervical epithelium in vivo

Exposure of the basement membrane by micro trauma allows PV to bind to Heparan Sulfate Proteoglycans (HSPG). Binding of PV to HSPG and subsequent interaction with host cell protein Cyclophilin B causes a conformational change in capsid structure resulting in the exposure of the N-terminus of L2 (Bienkowska-Haba et al., 2009). L2 has a furin cleavage motif and is cleaved by furin (Richards et al., 2006). Subsequently, virions are internalized into the basal cells to deliver the viral genome to their nucleus (Day et al., 2013) (A). Clustal analysis of the furin cleavage motif, R-x-K/R-R (boxed in black) on a variety of HPV and other commonly studied animal PV L2 (B). Clustal Analysis was performed with the UCSF Chimera package.

Figure 4. The putative transmembrane domain of L2

Clustal analysis of the transmembrane-like (TM-like) domain in L2 of fourteen PVs (A). Alpha helical modeling of the TM domains with the colors indicating the conserved GXXXG domains on each face (B) (Bronnimann et al., 2013). Images were provided by the courtesy of Samuel Campos, University of Arizona. Clustal Analysis was performed with the UCSF Chimera package.

Figure 5. Strong sequence conservation of 17-36aa region (recognized by the RG-1 neutralizing antibody) among different HPV types

Note the two cysteine residues (C22 and C28) in this region are important for infection and recognition by RG-1 (Campos and Ozbun, 2009; Gambhira et al., 2009). Clustal Analysis was performed with the UCSF Chimera package.